1. Field of the Invention
This invention relates to a lens position control device for a camera which is adapted for a video apparatus such as a video camera and the like.
2. Description of the Related Art
The recent advancement of video apparatuses such as video cameras, etc., is conspicuous. They have come to have an automatic focusing (hereinafter referred to as an AF) function and a zoom lens in general. The zoom lens which has a zooming function is formed in a lens system consisting of, for example, four lens groups as shown in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, the four-group lens system is arranged to permit a power varying operation and focus adjustment by changing the condition of the lens (such as shifting its position in the direction of the optical axis). The feature of the lens system varies according to the selection of lens groups that are to be movably arranged while others are arranged to be stationary.
In the case of FIG. 1, for example, a first lens group 1 is arranged to be stationary. A second lens group 2 is arranged to be a power varying zoom lens (hereinafter referred to as a V lens). A third lens group 3 (hereinafter referred to as a C lens) is provided for correcting a change of a focal plane resulting from a zooming action by correcting the image forming position and adjusting the focus. A fourth lens group 5 is arranged as an image forming system. A reference numeral 4 denotes an iris. A numeral 61 denotes the image sensing plane of an image sensor which is, for example, a CCD or the like.
FIG. 2 shows the positional relation obtained between the V lens and the C lens of the above-stated lens system when a zooming action is performed while keeping an in-focus state on the image sensing plane 61 for an object distance. In FIG. 2, the position of the V lens is shown on the axis of abscissa and that of the C lens on the axis of ordinate. The relation in position between the V and C lenses (hereinafter referred to as V-C curve) is shown with the object distance used as a parameter. The left end of the axis of abscissa indicates a point at which the shortest focal length is obtained (hereinafter referred to as the wide-angle end). The right end of the axis of abscissa indicates a point at which the longest focal length is obtained (hereinafter referred to as the telephoto end).
As is apparent from FIG. 2, the V-C curve changes the movement locus thereof according to the object distance. Therefore, an extremely complex mechanism is necessary if the V lens and the C lens are to be mechanically interlocked. To obtain means for correctly tracing the V-C curve with a simple mechanism, therefore, the following method is conceivable: The locus of each V-C curve is stored in an element such as a microcomputer or the like which is arranged to control the movement of the lens groups and the lens system is driven according to the stored values.
FIG. 3 diagramtically shows a method for storing the above-stated V-C curves in the microcomputer. The V-C curves vary their loci according to the object distance as mentioned above. However, the inclinations of adjacent V-C curves do not differ so much in most cases. Therefore, with the V-C curves divided into parts where the inclinations are about equal, the V-C curves shown in FIG. 2 can be divided into a plurality of discrete blocks as shown in FIG. 3. One representative inclination is assigned to each of the divided blocks. By this, the V-C curves of FIG. 2 can be discretely arranged.
Generally, the power varying speed of a zooming action is kept constant. Therefore, with this applied to FIGS. 2 and 3, the axis of abscissa can be replaced with a time base. Further, since the axis of ordinate indicates the degree of displacement of the C lens, the degree of the speed inclination given to each block can be treated as the speed of the C lens. Therefore, at the above-stated microcomputer, the moving or shifting speed of the C lens is stored for every one of the above-stated blocks. The speed computed on the basis of the V-C curve for each of the blocks is hereinafter called V.sub.CZ.
The microcomputer is arranged to always grasp, during its operation, the position of the V lens and that of the C lens. When a zooming action is performed, the microcomputer makes a check to find the block to which the positional relation between the V and C lenses applies. The lens position is shifted to pass through these blocks at speeds predetermined for the applicable blocks. This makes it possible to discretely trace the V-C curve. As a result, the V-C curve can be approximately traced in an arcuate manner as shown in FIG. 2.
The above is a typical example of the V-C curve tracing arrangement generally employed for a zoom lens of the kind shown in FIG. 1.
In accordance with the above-stated arrangement, the current lens position is determined either according to the speed assigned to each block that has been passed during the zooming action or according to the speed assigned to a block to which the lens position is located immediately before commencement of the zooming action. However, this arrangement has presented the following problems:
(1) Referring to FIG. 2, it is difficult to shift the lens position from one of the V-C curves to another. When the object distance changes during the zooming action, therefore, it is difficult to bring the lens system into an in-focus position again.
(2) In a case where the lens system is not perfectly in focus immediately before commencement of the zooming action, the zooming action is performed on the lens system, leaving it in a blurred state as focus adjustment is impossible during the zooming action.
(3) The focusing accuracy lowers on the side of the wide-angle end W which gives a depeer depth of field. On the wide-angle side, therefore, the lens system tends to be considered in focus even when the lens position is not exactly on the V-C curve. Then, if the lens position is shifted toward a telephoto end T by the zooming action in this state, the degree of blur comes to increase accordingly as the depth of field becomes shallower.
(4) In a case where the lens is brought to a stop during the process of the zooming action for some reason, the adverse effect of this continues to cause the lens position to be shifted over wrong blocks until the end of the zooming action.
Lens position control devices of the above-stated kind are disclosed in U.S. patent applications Ser. No. 340,025 filed on Apr. 18, 1989, now U.S. Pat. No. 4,975,714, Ser. No. 346,630 filed on May 2, 1989, now U.S. Pat. No. 4,920,369, Ser. No. 359,388 filed on May 31, 1989, now U.S. Pat. No. 4,950,054, Ser. No. 359,730 filed on May 31, 1989, Ser. No. 362,265 filed on June 6, 1989, Ser. No. 399,700 filed on Aug. 28, 1989, etc., which are assigned to the assignee of the present invention.